The invention relates to a method of manufacturing a scandate dispenser cathode having a matrix at least the top layer of which at the surface consists substantially of tungsten (W) and scandium oxide (Sc.sub.2 O.sub.3), and having emissive material in or below said matrix.
The invention also relates to a scandate dispenser cathode manufactured by means of this method.
The invention moreover relates to a method of manufacturing a powder of tungsten grains which are covered at least partly with scandium hydride (ScH.sub.2).
Such cathodes are used as an electron source in display tubes, camera tubes, oscilloscope tubes, klystrons, transmitter tubes, etc.
A characteristic of such dispenser cathodes is that there is a functional separation between on the one hand the electron emissive surface and on the other hand a store of the emissive material which serves to produce a sufficiently low work function of said emissive surface. One type of a dispenser cathode is the L-cathode. The emission of an L-cathode takes place from the surface of a porous matrix of, for example, tungsten, the work function of which is reduced by adsorbed barium (Ba) and oxygen (O). Below its matrix the L-cathode has a storage space in which a mixture of tungsten powder and emissive material, for example, barium-calcium aluminate, is present. The adsorbate at the surface is maintained by means of reactions of this mixture.
A second type of dispenser cathode is the impregnated cathode which is obtained by impregnating a compressed and sintered porous tungsten body with emissive material. In this case the required adsorbate is obtained by means of a reaction of the emissive material with the tungsten of the matrix.
A method of the type described in the opening paragraph is known from Netherlands Patent Application No. 8201371 (PHN 10,308) laid open to public inspection which may be considered to be incorporated herein. The advantages of the dispenser cathodes manufactured according to this known method are a good life and a reasonable to moderate recovery after ion bombardment.
It is therefore an object of the invention to provide a method of manufacturing a scandate dispenser cathode the recovery of which after ion bombardment is improved. Another object of the invention is to provide a cathode in which the scandium is distributed more homogeneously in the tungsten than in cathodes comprising scandium oxide grains.
Still a further object of the invention is to provide a method of manufacturing a powder consisting of tungsten grains which are covered at least partly with scandium hydride, which powder is of use in the manufacturing of a scandate dispenser cathode.
A method of the kind described in the opening paragraph is characterized according to the invention in that it comprises the following steps:
(a) compressing a porous plug of tungsten powder; PA1 (b) heating said plug in a non-reactive atmosphere and in contact with scandium to above the melting temperature of scandium; PA1 (c) cooling the plug in a hydrogen (H.sub.2) atmosphere; PA1 (d) pulverizing the plug to form fragments; PA1 (e) heating the fragments to approximately 800.degree. C. and firing at this temperature for a few to a few tens of minutes in a hydrogen atmosphere and then slowly cooling the fragments in hydrogen atmosphere; PA1 grinding the fragments the scandium hydride-tungsten (ScH.sub.2 /W) fragments to a powder; PA1 (g) compressing a matrix or a top layer on a matrix of pure tungsten from said ScH.sub.2 /W powder or from a mixture of said powder with tungsten powder; PA1 (h) sintering and cooling this matrix to form a cathode and PA1 (i) introducing emissive material into the cathode.
Experiments have demonstrated that a coating of the order of magnitude of a mono-layer of barium on bulk scandium oxide does not give rise to a high emission. Furthermore, the reaction of scandium oxide with tungsten and tungsten oxide is of importance for the oxygen concentration on the surface of the cathode. It is hence of importance to have scandium oxide in contact with tungsten. The use of scandium oxide grains does not seem the best solution for this purpose, because in fact the core of the grain will yet not contribute to the desired processes. By using the method according to the invention, the tungsten grains in the cathode surface are partly covered with scandium oxide or scandium having scandium oxide thereon. As a result, a more homogeneous distribution of scandium over the cathode surface is also obtained than is the case when a mixture of scandium oxide grains and tungsten grains is used.
The compression of a porous plug of tungsten powder (step a) is carried out, for example, to a density of approximately 60% of the density of tungsten metal.
Heating the plug (step b) is done in a non-reactive atmosphere, but preferably in a vacuum, because then a good coating of the tungsten with scandium is obtained. Thus coating is obtained by heating the plug in contact with scandium to above the melting temperature of scandium, as a result of which the melted scandium is drawn into the porous plug. The scandium may be provided on the plug, for example, in the form of a lump of scandium. For example, approximately 3% by weight of scandium is taken up in the plug. The plug is then cooled in hydrogen (step c) as a result of which it becomes brittle due to the fact that the scandium is partly converted into scandium hydride, so that an increase in volume occurs. The plug is then pulverized to form fragments (step d). The fragments are then heated in a molybdenum crucible in a hydrogen atmosphere up to 800.degree. C. and kept at this temperature for approximately 15 minutes and slowly cooled in the same hydrogen atmosphere, substantially all the scandium being converted into scandium hydride (step e). The fragments are then ground in an agate mill to grains of the desired size (step f). Since scandium hydride is a stable compound the resulting powder may hence be stored in air.
Upon sintering a cathode matrix, the scandium hydride is decomposed (above 800.degree. C.). Because scandium hydride has a larger specific volume than scandium, it is therefore to be preferred upon sintering and cooling in hydrogen, to remove the hydrogen at a temperature above 800.degree. C. by pumping. Upon sintering in a vacuum this problem does, of course, not occur. However, in that case special measures must be taken to avoid excessive scandium evaporation. It is possible indeed upon sintering and cooling in hydrogen to obtain a good result when the powder manufactured in step (f) is provided as a top layer on the tungsten matrix, in particular when said powder is dehydrated or is mixed with 25 to 75% by weight of tungsten powder, preferably approximately 50% by weight of tungsten powder. Such a top layer preferably has a thickness which is smaller than 0.15 mm. As an impregnant in the cathodes to be described hereinafter, a conventional barium-calcium aluminate has been used. The whole or partial oxidation of the scandium present on the tungsten grains takes place during the manufacture of the cathode, for example, upon impregnating and/or activating. It is to be noted in this connection that scandium oxide thermodynamically is stabler than barium oxide.